Alignment marks are important in fabrication of semiconductor, or integrated circuit (“IC”), devices because the devices are produced by aligning several layers of conductive, semiconductive, and insulative materials one atop the other with reference to the alignment marks. It is critical that each layer is precisely aligned with the previous layer so that the resultant circuits are functional and reliable. Typically, aligning a layer with a previous one is accomplished using a wafer stepper, which is used to optically project a circuit pattern on a mask mounted therein onto a layer of the wafer disposed on a wafer chuck of the stepper. Before the mask pattern is transferred, the wafer must first be precisely aligned with the mask. Once such alignment is achieved, the remaining steps of projecting the mask pattern on to the semiconductor may be performed.
During the alignment phase, the position of the alignment mark on the wafer is typically sensed using a laser beam, which is bounced off the alignment mark to produce a reflective light signal. This reflective light is reflected back to an inspector of the stepper. The stepper analyzes the reflected light to determine the exact position of the alignment mark. Notably, the quality of the signal reflected from the alignment mark is directly dependent on the reliability and integrity of the structure thereof. Alignment marks fabricated using existing technologies do not lead to a strong reflected signal, which makes accurate alignment more difficult.
Therefore, while existing methods of forming alignment marks have been generally adequate for their intended purposes, they have not been entirely satisfactory in every aspect.